The invention concerns a weigh module for installation in a weighing scale of the kind where a weighing load carrier such as for example a platform, a tank, a hopper, or a belt conveyor is supported by a plurality of weigh modules, wherein each weigh module contains a load cell and the weight of a load on the weighing load carrier is determined by summation of the individual load cell signals.
A conventional weigh module includes a base part that can be bolted to a foundation or support structure, a top part that can be bolted to the weighing load carrier, a load cell that is fastened to the base part, and a force-transmitting member which is arranged between the top part and the load cell and serves to introduce the weighing force from the top part into the load cell. The function of the weigh module is to ensure that only the substantially vertical weight force that is to be measured is received by the load cell, while laterally directed forces and/or relative displacements between the top part and the base part which could harm the load cell or compromise the weight measurement are prevented from reaching the load cell.
In an installation of this kind, lateral forces or relative displacements between the top parts and the bases of the weigh modules can arise for example as a result of different thermal expansions of the weighing load carrier and the foundation or support structure, deformation of the weighing load carrier caused by the weighing load, seismic shifts, wind forces, braking and acceleration of vehicles on a vehicle scale, and other forces originating from inside or outside the weighing scale.
To protect the load cell from having to absorb lateral forces and/or displacements, the force-transmitting member may be configured, for example, as a so-called rocker pin, a construction element generally known to those of ordinary skill in the relevant art. Arranged in vertical orientation between the top part and a load-introduction area or -element of the load cell, a rocker pin transmits a force along its longitudinal axis while essentially yielding to transverse forces and displacements of the top part, resisting such forces and displacements only through its inherent but very limited capability to maintain and/or restore its vertical orientation. Besides rocker pins, the state of the art also includes force-transmitting members in which a self-righting resilience is achieved in different ways, for example by inserting elastomeric pads in the contact zones where the upper and lower end of the force-transmitting member meet, respectively, the top part of the weigh module and the force-introduction zone of the load cell.
To absorb transverse forces and displacements that exceed the limited position-restoring ability of the force-transmitting member, weigh modules normally include means to restrict or constrain the freedom of lateral movement of the top part in relation to the base part. A movement-restricting means is generally of the nature of a solid boundary which confines movement of the top part relative to the base part in any horizontal direction to a limited zone of free play and is strong enough to withstand horizontal forces within given design specifications.
The restricting means may further be designed with the capability to stop vertical upward movement of the top part and thus to prevent the weighing load carrier (which is fastened to the top part) from being lifted off the base, for example by wind forces. Finally, the restricting means may include a stop to prevent an excessive downward drop of the weighing load carrier in case of a mechanical collapse of the load cell.
In addition to the restricting means in the form of movement-stopping boundaries, a weigh module may also include constraints, for example in the form of horizontal checkrods linking the top part to the bottom part and thereby inhibiting any linear movement of the top part relative to the base part in the direction of the checkrod.
However, in an arrangement of weigh modules that are thus constrained in one or two horizontal directions, care must be taken to avoid over-constraints. For example, a platform scale with four weigh modules could have three weigh modules with one checkrod each and a fourth weigh module without checkrod, arranged in such a way that no two of the checkrods are collinear with each other.
The actual arrangement and configuration of the aforementioned generic elements in a weigh module depends to a large extent on the type of load cell, the primary distinction being between so-called canister load cells and cantilever beam load cells. A canister load cell generally has the shape of an upright cylinder whose bottom is fastened to the base plate of the weigh module. The weighing force is directed substantially along the axis of the cylinder and is applied to a so-called load button at the center of the top surface of the upright cylinder.
A variant of the canister load cell, the so-called rocker pin load cell likewise has the form of an upright cylinder. However, both the top and bottom are spherically shaped rocker surfaces. Installed in a weigh module, this load cell is arranged like a rocker pin between the top part and the base part of the weigh module. In other words, the rocker pin load cell combines the functions of load cell and force-transmitting member in one integral unit.
In contrast to the canister load cell or the rocker pin load cell, a cantilever beam load cell is basically configured as a horizontal beam which, when installed in a weigh module, is at one end solidly connected to the base of the weigh module while the opposite, free end of the cantilever beam receives the weighing load from the top part of the weigh module by way of a force-transmitting member.
The following examples of the state of the art illustrate specific embodiments of the foregoing generic concepts of weigh modules.
A weigh module according to U.S. Pat. No. 6,331,682 has a base part and a top part in the form of flat plates with mounting holes for their attachment to a foundation or support structure and to a weighing load carrier such as a tank or a platform. The load cell in this weigh module is configured as a canister load cell, and the force-transmitting member is configured as a rocker pin. Movement-restricting means are provided in the form of four lower block-shaped posts rising up from the base plate on opposite sides of the load cell and two upper block-shaped posts descending downward from the top plate into interstices between the lower block-shaped posts, leaving lateral play between the upper and lower block-shaped posts. Horizontally oriented restraining pins are held with a tight fit in seating holes in the lower block-shaped posts and pass with all-around free play through clearance holes in the upper block-shaped posts. Movement of the top plate relative to the base plate is thus restricted: a.) in a horizontal x-direction by the horizontal play between pins and clearance holes; b.) in a horizontal y-direction by the lateral play between the upper and lower block-shaped posts; and c.) in a vertical z-direction (upward and downward) by the vertical play between pins and clearance holes. With more than one movement-stopping contact area in each direction, this weigh module has a design redundancy that does not appear to be justifiable. When the weigh module is pushed laterally in any given direction, the respective stop contacts cannot be expected to engage simultaneously, and their redundancy will therefore not increase the capability of the weigh module to withstand lateral forces. Also, visual inspection of all clearances appears to be rather cumbersome, and the horizontal passage holes in the block-shaped posts may be prone to collect dirt and debris which could compromise the weighing accuracy. In addition, the fabrication and assembly of six block-shaped posts and two pins gives the impression of being relatively expensive to realize in practice.
A weigh module described in U.S. Pat. No. 3,997,014 has a base part in the form of a flat plate and a top part in the form of an angle profile with a horizontal and a vertical portion, with the vertical portion having mounting holes through which the top part can be bolted sideways to a vertical surface of the weighing load carrier. The load cell in this weigh module is configured as a rocker pin load cell which performs the combined functions of load sensor and force-transmitting member as described hereinabove. Movement-restricting means are provided in the form of two vertical posts rising up from the base plate on opposite sides of the load cell and by clearance holes in the horizontal portion of the angular top part, wherein the upper extremities of the vertical posts reach into the clearance holes with lateral play, thus performing the lateral movement-restricting function. Like the preceding example, this weigh module again has redundant movement-stopping elements whose contributions in absorbing horizontal forces may not necessarily be additive. Besides, free-standing cylindrical posts do not appear to be the most efficient design for the absorption of transverse forces.
A weigh module according to DE 199 18 408 A1 has a base part and a top part in the form of flat plates, with a rocker pin load cell transmitting and simultaneously measuring the weighing force between the top plate and the base plate. Also mentioned in this reference is the possibility of using elastomeric bearings rather than spherical rocker surfaces for the introduction of the weighing force and its equal and opposite reaction force into the load cell. A first movement-restricting part is provided in the form of a massive cylindrical bumper post rising up from the base plate and carrying at the top a rectangular plate. A second movement-restricting part has the form of an angular profile descending from the top plate. The corner ends of the rectangular plate reach into cutouts of the angular profile, wherein the clearances between the projections and the edges of the cutouts delimit the free movement of the top part relative to the base part in an x-direction towards (but not away from) the load cell and both ways in the y-direction perpendicular to the x-direction. Full restriction of horizontal movement in all directions is achieved by orienting the individual way modules of an installation appropriately, so that their respective movement-blocking capabilities complement each other. An area of concern with this weigh module would be the cost and feasibility of connecting the massive post to the base plate, the rectangular plate to the post, and the angular profile to the top plate either by welding or by means of screw bolts.
An example of a weigh module with a lateral constraint in the form of one horizontal checkrod is illustrated in FIG. 2 of DE 101 38 435 A1. By linking the top part to the bottom part, the checkrod inhibits linear movement of the top part relative to the base part in the direction of the checkrod. Horizontal movements transverse to the direction of the checkrod are limited within a range of free play between a first and a second part of a movement-restricting means. As mentioned above, by arranging three such weigh modules with their checkrods oriented along non-collinear axes, it is possible to build a weighing scale without loose play between the weighing load carrier and the supporting base while at the same time avoiding over-constraints that would introduce lateral forces into the load cells. In a scale with more than three weigh modules, one would simply omit the checkrod from the fourth and any further weigh modules used in the installation. This weigh module, too, suffers from the drawback of being complicated and expensive to realize in practice. A welded table-like structure which forms a first movement-restricting part rises from the base plate to within a narrow distance of the top plate, while separate holder brackets for the bearings of the checkrod are also welded to the base plate and the top plate, respectively.